Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers. Here and elsewhere, it will be understood that the term “light” is used to encompass electromagnetic radiation regardless of whether it is within the visible part of the spectrum.
Methods for generating EUV light include converting a source material from a liquid state into a plasma state. The source material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV part of the spectrum. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a laser beam to irradiate a source material having the required line-emitting element.
One LPP technique involves generating a stream of source material droplets and irradiating at least some of the droplets with laser light. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source material having at least one EUV emitting element, such as xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV.
The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (an example of an EUV optic referred to as a collector or collector mirror) is positioned to collect, direct (and in some arrangements, focus) the light to an intermediate location. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer.
In some LPP systems each droplet is sequentially illuminated by multiple light pulses. In some cases, each droplet may be exposed to a so-called “pre-pulse” and then to a so-called “main pulse.” It is to be appreciated, however, that the use of a pre-pulse is optional, that more than one pre-pulse may be used, that more than one main pulse may be used, and that the functions of the pre-pulse and main pulse may overlap to some extent.
In quantitative terms, one arrangement that is currently being developed with the goal of producing about 100 W at the intermediate location contemplates the use of a pulsed, focused 10-12 kW CO2 drive laser which is synchronized with a droplet generator to sequentially irradiate about 10,000-200,000 tin droplets per second. For this purpose, there is a need to produce a stable stream of droplets at a relatively high repetition rate (e.g., 10-200 kHz or more) and deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position over relatively long periods of time.
For LPP light sources, it may be desirable to use one or more gases in the chamber for ion-stopping, debris mitigation, optic cleaning and/or thermal control. In some cases these gases may be flowing, for example, to move plasma generated debris, such as vapor and/or microparticles in a desired direction, move heat toward a chamber exit, etc. In some cases, these flows may occur during LPP plasma production. For example, see U.S. Pat. No. 7,671,349, issued on Mar. 2, 2010, the entire contents of which are hereby incorporated by reference herein. Other setups may call for the use of non-flowing, i.e., static or nearly static, gases. The presence of these gasses, whether static or flowing and/or the creation/existence of the LPP plasma may alter/effect each droplet as it travels to the irradiation region adversely affecting droplet positional stability. This may reduce dose performance and hence output power.
In U.S. Pat. No. 7,872,245, issued on Jan. 18, 2011, the entire contents of which are hereby incorporated by reference herein, the use of a tube to envelop a portion of the droplet path as the droplets travel from a droplet release point to an irradiation region was described. As described, the tube was provided to shield and protect an EUV optic from droplets/target material that strayed from the desired path between a droplet release point and the irradiation region, e.g. during droplet generator startup or shutdown.
U.S. Pat. No. 8,263,953, issued Sep. 11, 2012, the entire contents of which are hereby incorporated by reference herein, discloses an arrangement in which gas flows in a direction toward the droplet stream and a shroud is positioned along a portion of the stream, the shroud having a first shroud portion shielding droplets from the flow.
With the above in mind, applicants disclose systems and methods for target material delivery protection in a laser produced plasma EUV light source, and corresponding methods of use.